While erythropoietin has been purified by standard biochemical methods, the quantities obtained have been exquisitely small and inadequate for studies of the mechanism of action of this hormone. We have proposed purification of erythropoietin employing certain structural features, especially its high carbohydrate content. Crude sheep erythropoietin, obtained from commercial sources, and human erythropoietin in the urine of patients with hemolytic disorders have been used as material sources. Erythropoietin activity, as measured in the in vivo mouse exhypoxic assay may be increased 75-100 fold by specific binding to a lectin affinity column. Activity may be further purified by isoelectric focusing on sucrose density gradients. As assay system based on the supportive role of erythropoietin for erythroid cells in culture is being developed. Partially purified erythropoietin may serve as an immunogen for fractionation of specific antibodies to erythropoietin and may be the basis of large scale purification of this hormone.